Fuel efficiency of vehicles has been increased by reducing the rolling resistance of tires to suppress heat build-up. In recent years, such contributions of tires to an increase in fuel efficiency have been increasingly demanded. There has previously been a great need to improve the fuel efficiency of treads, among other tire components, which constitute a large portion of a tire. More recently, there has been a need not only for treads but also for sidewalls, insulation components, breaker cushions, and the like to have higher fuel efficiency.
Known approaches to improve the fuel efficiency of rubber compositions include the use of low reinforcing filler, or the use of a smaller amount of reinforcing filler. Another known approach for improving fuel efficiency is to use silica filler to reduce rolling resistance. However, these approaches reduce the reinforcing properties of rubber compositions, and therefore disadvantageously reduce breaking performance, such as flex crack growth resistance, and abrasion resistance.
In particular, tires for trucks and buses are used under very severe conditions, and thus treads of tires for trucks and buses require high abrasion resistance as well as high breaking performance sufficient to prevent chipping of treads, etc. To satisfy this requirement, the compositions for treads of tires for trucks and buses typically contain natural rubber and/or polybutadiene rubber. There have been attempts in recent years to further improve abrasion resistance by increasing the cis content or the molecular weight of polybutadiene rubber, or by subjecting polybutadiene rubber to modification for carbon black. In contrast, unlike polybutadiene rubber, there is at present very little development of techniques for natural rubber, which is a main component of the composition for treads of tires for trucks and buses, because it is a natural product.
Accordingly, the development of a natural rubber having high abrasion resistance and high breaking performance has been required.
Natural rubber is mainly formed of polyisoprene. Unlike synthetic polyisoprenes, it has a high gel fraction. The term “gel fraction” refers to a fraction poorly soluble in a solvent. It is believed that the gel fraction is derived from branched structures formed by large amounts of protein, lipids and the like contained as impurities in natural rubber. In fact, it is known that the gel component is reduced to some extent by deproteinization to remove proteins as allergenic substances.
For example, Patent Literatures 1 and 2 disclose methods of reducing proteins and the like contained in natural rubber by adding a proteolytic enzyme and a surfactant to natural rubber latex, and aging the mixture. Besides, in order to reduce the gel content in natural rubber, Patent Literature 3 discloses a method of immersing a solid natural rubber swollen with a solvent in an alkali hydroxide; Patent Literature 4 discloses a method of adding a phosphate to natural rubber latex and then removing magnesium phosphate; and Patent Literature 5 discloses a method of adding a surfactant to natural rubber latex followed by washing. Moreover, Patent Literature 6 discloses a method of preparing a natural rubber having a low Mooney viscosity by treating natural rubber latex with a phospholipase or lipase. Furthermore, Patent Literature 7 discloses a method for coagulation of natural rubber using a polymer flocculant.